Surface Flatness Calculation

[kv802]

I leveled to, and moved the origin to a 10 point plane on a surface I am measuring.
Next, I probed 40 points on the surface and measured the 'Z' location of each point.
Next, I constructed a BFRE plane from the 40 points and measured the flatness of the 40 point plane.

Max Z of the 40 points: 0.0407mm
Min Z of the 40 points: -0.0610mm Range between Max and Min: 0.1017mm
Flatness: 0.0953mm

I was expecting the Flatness output to be the same as the Max-Min range.
Does anyone know why these results are different?

Maybe this is a question for tech support, but I was wondering if someone could email me the math on how flatness is calculated.
PC-Dmis v3.7 MR2

Thanks,
Keith

[terry]

I'll bet if you re level to your 40 point plane and then ask for the min max the range and your flatness will be the same. I believe that when you ask for flatness in PC-DMIS it relevels the surface so the leveling plane between 10 points and 40 points is most likely diffrent so the min/max will be diffrent because the leveling plane (or average surface) would be "skewed" diffrently.
Terry

[Matthew D. Hoedeman]

Flatness has ABSOLUTELY NOTHING to do with LOCATION, it is a free-floating dimension, it does not care WHERE the plane is, but the individual check points are to your alignment.

[Paul Barriage]

I have also experienced FLATNESS as a doubling feature, like true position while, as Matt says, it has nothing to do with location. It just doubled on one I checked. I measured it as surface profile points.

[ag162]

Flatness = Total range if I'm not mistaken.
Ex: Z -.003mm, Z .027mm = Total .03mm Flatness

VP A.Gore

[Matthew D. Hoedeman]

Well, what Pcdmis does when checking flatness is it will do a small internal alignment based only on the plane, basically, it levels to the plane, then the highest to the lowest deviation from the plane 'nominal' is the flatness reported.

[ag162]

Exactly as Matt and Terry stated.

VP A.Gore

[terry]

Isn't that what I said?

[Matthew D. Hoedeman]

Yeah, but I told him what Pcdmis is doing in the background to come up with the number it was giving him. Basically, backing up what you were telling him, but by describing how Pcdmis came up with the number it came up with.

[Jonathan S]

Agree. It wasn't necessary to level to the 40 point plane because Pcdmis does that automatically in the calculation of flatness, but as Terry said, if you do level to it then you should see the same numbers.